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We have seen that development is influenced by the dynamic interaction of biological and contextual forces. Genes provide a blueprint for development, but phenotypic outcomes, individuals’ characteristics, are not predetermined. Our genes are expressed as different phenotypes in different contexts or situations, known as epigenetics (Moore, 2017). The term epigenetics literally means “above the gene.” The epigenome is a molecule that stretches along the length of DNA and provides instructions to genes, determining how they are expressed, whether they are turned on or off. The epigenome carries the instructions that determine what each cell in your body will become, whether heart cell, muscle cell, or brain cell, for example. Those instructions are carried out by turning genes on and off.

At birth, each cell in our body turns on only a fraction of its genes. The epigenome instructs genes to be turned on and off over the course of development and also in response to the environment (Meaney, 2017). Epigenetic mechanisms determine how genetic instructions are carried out to determine the phenotype (Lester, Conradt, & Marsit, 2016). Environmental factors such as toxins, injuries, crowding, diet, and responsive parenting can influence the expression of genetic traits. In this way, even traits that are highly canalized can be influenced by the environment.

One of the earliest examples of epigenetics is the case of agouti mice, which carry the agouti gene. Mice that carry the agouti gene have yellow fur, are extremely obese, are shaped much like a pincushion, and are prone to diabetes and cancer. When agouti mice breed, most of the offspring are identical to the parents—yellow, obese, and susceptible to life-shortening disease. However, a groundbreaking study showed that yellow agouti mice can produce offspring that look very different (Waterland & Jirtle, 2003). The mice in the photo both carry the agouti gene, yet they look very different; the brown mouse is slender and lean and has a low risk of developing diabetes and cancer, living well into old age. Why are these mice so different? Epigenetics. In the case of the yellow and brown mice, the phenotype of the brown mice has been altered, but the DNA remains the same. Both carry the agouti gene, but it is turned on all the time in the yellow mouse and turned off in the brown mouse.

These two mice are genetically identical. Both carry the agouti gene, but it is turned on all the time in the yellow mouse and turned off in the brown mouse.

Attribution 3.0 Unported (CC BY 3.0)

In 2003, Waterland and Jertle discovered that the pregnant agouti female’s diet can determine her offspring’s phenotype. In this study, female mice were fed foods containing chemicals that attach to a gene and turn it off. Yellow agouti mothers fed extra nutrients passed along the agouti gene to their offspring, but it was turned off. The mice looked radically different from their mother (brown) and were healthier (lean, not susceptible to disease) even though they carried the same genes.

Epigenetic processes also influence human development. For example, consider brain development. Providing an infant with a healthy diet and opportunities to explore the world will support the development of brain cells, governed by epigenetic mechanisms that switch genes on and off. Brain development influences motor development, further supporting the infant’s exploration of the physical and social world and thereby promoting cognitive and social development. Active engagement with the world encourages connections among brain cells. Conversely, epigenetic changes that accompany exposure to toxins or extreme trauma might suppress the activity of some genes, potentially negatively influencing brain development and its cascading effects on motor, cognitive, and social development. In this way, an individual’s neurological capacities are the result of epigenetic interactions among genes and contextual factors that determine his or her phenotype (Lerner & Overton, 2017). These complex interactions are illustrated in Figure 2.10 (Dodge & Rutter, 2011). Interactions between heredity and environment change throughout development, as does the role we play in constructing environments that support our genotypes, influence our epigenome, and determine who we become (Lickliter & Witherington, 2017).

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Figure 2.10 Epigenetic Framework

Source: Gottlieb (2007). With permission from John Wiley & Sons.

Perhaps the most surprising finding emerging from animal studies of epigenetics, however, is that not only can the epigenome be influenced by the environment before birth but it can be passed by males and females from one generation to the next without changing the DNA itself (Soubry, Hoyo, Jirtle, & Murphy, 2014; Szyf, 2015). This means that what you eat and do today could affect the epigenome—the development, characteristics, and health—of your children, grandchildren, and great-grandchildren (Bale, 2015; Vanhees, Vonhögen, van Schooten, & Godschalk, 2014).